TID looks to avert tunnel collapse

Canal shutdown possible without fix

The Turlock Irrigation District is set to blast away dirt and rock covering a 360-foot long tunnel on its upper main canal, “daylighting” the canal and removing the top of the tunnel in hopes of preventing a potentially disastrous tunnel collapse.

Should the tunnel fail, it could result in a complete shutdown of the TID irrigation system.

The 360-foot long, 20-foot wide tunnel was originally built in 1891, and then widened to 30-feet in 1914. The concrete base and concrete lined walls withstood the test of time, but the unsupported rock ceiling has begun to collapse.

A 40-foot long by 25-foot wide cavity has developed roughly mid-tunnel, stretching 17 feet above the former tunnel crown. In 2003 a further 15-foot by 15-foot by 4-foot slab fell from the roof into the canal, mid-summer, at the height of canal flows.

Concerns regarding further collapse led TID directors to authorize a $200,000 investigation into the tunnel, completing seismic analysis, exploratory drilling, and the initial phase of project planning.

“We were hoping it was localized to the area of the crown and wasn’t widespread like it is,” said TID Senior Civil Engineer Brad Koehn.

But the weaker material – a gravel conglomerate, rather than the firmer sandstone making up much of the tunnel – was found to be extensive, putting the tunnel in danger of potential collapse at an unknown future date.

“The answer is, we’re not sure if it’s going to collapse,” Koehn said. “The only thing we can do is learn everything we can about the tunnel, its integrity, and the risks.”

Should the entire tunnel collapse, the Turlock Irrigation District’s upper main canal system could be shut down. Water already in the system at the time of collapse could overflow, wiping out nearby homes and washing out large sections of canal.

Assuming usage similar to normal conditions, a collapse would leave TID with enough water for about seven days of irrigation – just that within Turlock Lake. A collapse resulting in a washout of the nearby Delaney Field canal section could result in a two to three month shutdown of the irrigation system.f

“What kind of plan do you have in effect if the tunnel goes out tomorrow?” Alamo asked staff.

In truth, TID has plans, reliant upon rationing the water in Turlock Lake – resulting in delays for water orders – and renting hundreds of groundwater pumps. In the dry 1988 water year, when TID rented 300 pumps and pumped 300,000 acre-feet of groundwater, the action had unintended side effects, either reducing water quality or eliminating water to over 300 domestic well users.

Per a 1997 TID report, a month-long outage in the heart of irrigation season could cost TID $850,000 – not counting the costs of repairing the canal system. To farmers, a 30 day irrigation outage would result in nearly $14 million in lost revenue.

Tunnel repair to be costly, may cost customers

Daylighting the tunnel – blasting away dirt and sand to remove the top of the tunnel – is projected to cost the district about $975,000. The month-plus long project will require 14 to 16 straight days just of moving dirt and rock – nearly 80,000 square yards.

Due to the lengthy environmental process the project won’t be completed until next fall, during an irrigation downtime. But the district will file a notice of intent to perform the project, per environmental law, on Friday, with the TID Board of Directors due to issue final project approval on Sept. 13.

Preliminary planning will be paid for with unused funding from this year’s irrigation capital projects budget. TID General Manager Casey Hashimoto said he expects the district will maintain the current budget, but reprioritize projects to ensure the project is completed over work like new irrigation drops and gunnite canal repair.

With the district already behind on its gunnite work, TID Director Michael Frantz suggested the district may not be able to continue to put off the less-important tasks, and may ask growers for “some sort of a revenue increase” to cover the direct costs of repairing and improving the canal system.

“I think it’s a worthwhile, worthy cost,” Frantz said.

TID considers longer irrigation season

The Turlock Irrigation District said Tuesday it may extend the irrigation season to Nov. 2, due to the unusual growing season.

The season was originally projected to end Oct. 19.

Corn and walnut growers in particular were concerned with that end date, given the delayed growing season. Some corn farmers are still planting crops, TID staff said.

The district extended the irrigation in 2010 for similar reasons, adding 11 days to a season which concluded Oct. 31, 2010.

The extended irrigation season hinges on weather conditions. Should temperatures drop and rain begin falling – reducing grower interest in irrigation – the district may close the season earlier.